The present invention relates to telecommunications in general, and, more particularly, to an architecture for a wireless telecommunications system.
FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (xe2x80x9cWSCxe2x80x9d) 120, which may also be known as a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) or a Mobile Telephone Switching Office (xe2x80x9cMTSOxe2x80x9d). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called xe2x80x9ccells.xe2x80x9d As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120 via wireline 102-1. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 104-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
When wireless telecommunications system 100 is a terrestrial system, in contrast to a satellitebased system, the quality and availability of service is subject to the idiosyncrasies of the terrain surrounding the system. For example, when the topography of the terrain is mountainous, or when buildings or trees are present, then signals transmitted by a wireless terminal can arrive at an antenna at the base station both directly and reflected off of objects in the vicinity of the base station. If two or more signals (e.g., one direct path signal and one or more reflected signals, two or more reflected signals, etc.) arrive at the antenna out of phase, then the signals can destructively interfere, which hinders the base station""s ability to acquire and process the signal. This phenomenon is known as multipath fading. Empirically, multipath fading is a highly localized phenomena such that if multipath fading occurs at one location, it is highly unlikely to occur at a location just a short distance away.
FIG. 2 depicts a block diagram of a typical base station in the prior art, which typically contains two receive antennas, Rx1 and Rx2, that are configured to serve the same geographic area and to capture variations of the same information-bearing signal. When the two antennas are positioned close to each other (e.g., within xc2xd wavelength of the information-bearing signal of each other), then it is likely that both antennas will capture variations of the information-bearing signal that have a similar signal strength. For example, if one of the antennas receives a variation of the information-bearing signal that is weak due to multipath fading, then it is likely that the other antenna will also receive a variation of the information-bearing signal that is weak.
In contrast, if the two antennas are positioned far from each other (e.g., more than several wavelengths of the information-bearing signal from each other), then it is unlikely that both antennas will capture variations of the information-bearing signal that have a similar signal strength. In other words, it is unlikely that both antennas will, at the same time, capture variations of the information-bearing signal that are weak because of multipath fading. Therefore, it is for this reason that many base stations employ two or more receive antennas to ensure that at least one variation of the information-bearing signal is captured that is strong and available for processing. The technique for employing N receive antennas to provide robustness in receiving information-bearing signals is known as N-way receive diversity.
When a base station employs N-way receive diversity, the base station incorporates an apparatus known as a diversity combiner to combine the variations of the demodulated information-bearing signal to create an estimate of the information-bearing signal that is better than the estimate that could be made if only one antenna was used. As is well-known in the prior art, the diversity combiner can use a variety of techniques (e.g., traditional selection diversity, equal-gain combining diversity, maximum-ratio combining diversity, etc.) to process the N variations of the information-bearing signals.
The principal disadvantages of a base station architecture that supports N-way receive diversity is that it substantially increases the cost and size of the base station by requiring N receive antennas, N radios for each information-bearing signal to be demodulated, and a diversity combiner in each base station. Furthermore, while the added cost is often justified in macrocellular systems in which a single base station services hundreds of wireless terminals, the added cost is typically prohibitive for indoor and microcellular systems. And, still furthermore, the added radios and diversity combiner in each base station only adds to the amount of equipment that can break-down in a base station and require expensive service calls.
Therefore, the need exists for a wireless telecommunications system architecture that provides the robustness associated with N-way receive diversity; techniques without the costs and disadvantages associated with solutions in the prior art.
The present invention is a wireless telecommunications system that provides the robustness associated with N-way receive diversity without some of the costs and disadvantages associated with techniques in the prior art. In particular, some embodiments of the present invention are capable of achieving the advantages of receive diversity with one receive antenna per cell, without multiple radios per cell and without a diversity combiner in each cell. This is advantageous because it greatly reduces the cost and complexity of a wireless telecommunications system.
Furthermore, some embodiments of the present invention are capable of allowing two or more receive antennas to share a single wireline, which can reduce the amount of cabling necessary to interconnect the various elements of the wireless telecommunications system.
And still furthermore, some embodiments of the present invention are capable of interconnecting the various elements of the system with inexpensive and easily-installed wireline (e.g., twisted-pair, etc.) in contrast to co-axial cabling.
In an illustrative embodiment of the present invention, the functionality performed by multiple base stations in the prior art is performed by multiple, geographically-dispersed radio heads and a shared, centralized baseband unit. Typically, each radio head comprises one receive antenna and the baseband unit comprises the equipment for demodulating and diversity combining the various information-bearing signals received by the radio heads.
Each radio head captures all of the radio-frequency information-bearing signals transmitted from the wireless terminals within a cell, downconverts them to intermediate frequencies, without demodulating them, and transmits them to the baseband unit. The baseband unit channel decodes, demodulates, demultiplexes, and combines the information-bearing signals to produce the respective traffic channels.
When a wireless terminal is located near a radio head, the information-bearing signal from that wireless terminal is typically received with sufficient power to be adequately received even if the receive antenna is in a fade when the signal is received. In contrast, when a wireless terminal is located near the boundary of a cell, the information-bearing signal from that wireless terminal is typically received by receive antennas at two or more adjacent radio heads. In this case, each radio head transmits its captured version of the information-bearing signal to the baseband unit, which having multiple versions of the information-bearing signal is capable of performs diversity combining on the versions.
An illustrative embodiment of the present invention comprises: a first antenna for receiving a first information-bearing signal at a first radio frequency; a first downconverter for downconverting the first information-bearing signal to a first intermediate frequency; a first transmitter for transmitting the first information-bearing signal at the first intermediate frequency over a first wireline; means for receiving the first information-bearing signal at the first intermediate frequency from the first wireline and for demodulating the first information-bearing signal.